Micromachining has been used to produce neural-electronic interfaces for recording neural activity with cortical probe arrays and planar arrays to form individual cells and clusters of cells. Recently, however, new applications that require the stimulation of neural tissue push the performance limits of conventionally produced microelectrodes. One good example is the development of a visual prosthesis. Blind patients with retinits pigmentosa or macular degeneration (i.e., photoreceptors do not function) observe a visual percept induced by the direct electrical stimulation of the retina. Internationally, several efforts are under way to construct a fall visual prosthetic system.
Examples of implantable nerve stimulators that benefit from very small electrodes are U.S. Pat. No. 5,109,844 (“De Juan”), U.S. Pat. No. 5,935,155 (“Humayun”), and U.S. Pat. No. 5,531,774 (“Schulman”). De Juan and Humayun disclose systems for the electrical stimulation of the retina by a retinal electrode array held against the retina. DeJuan describes an epiretinal electrode array. Humuyan describes a system for capturing a video image, transferring the image wirelessly into a living body and applying the image to a retinal electrode array. Schulman discloses a cochlear stimulator for the deaf.
While small electrodes help create a precise signal to stimulate a single nerve or small group of nerves, the ability of an electrode to transfer current is proportional to its surface area. It is further known that electrical signals transfer more efficiently from an edge than from a flat surface. It is, therefore, desirable for an electrode to have a large surface area with many edges.
One technological challenge is the trade off between electrode density and stimulation current. Although the total charge injection required to illicit a visual percept is generally believed to be fixed, the current used is limited by the electrode area when operated at the maximum current density before undesirable and irreversible electrochemical reactions occur. The result is that the electrode size and density in existing retinal prosthetic development efforts are limited.
Therefore, it is advantageous to increase the total surface area of an electrode without increasing the planar surface area. This can only be accomplished by increasing the vertical surface area. One method of increasing surface area is to roughen the surface by rapid electroplating. This is commonly done with platinum electrodes and known as platinum black as described in U.S. Pat. No. 4,240,878 (“Carter”). Platinum black is not very strong and tends to flake off. This is unacceptable in a neural stimulator. The flaking can be limited by ultrasonic vibrations as described in U.S. Pat. No. 4,750,977 (“Marrese”), but not eliminated. A system is needed to create a high surface area electrode which is strong enough for human implantation.